Figure 28.26 « Oxidation h^^^n''^"^ H3C^^N^|fN"H ^C^^-^N Nv and reduction of the isoallox- ° H O H O
azine rings. oxidized species reduced species alkaloid, nicotine. Niacinamide, also known as nicotinamide, refers to the amide derivative of niacin that is equivalent in vitamin activity. Some texts use niacin to refer to nicotinic acid, niacinamide, and any derivatives with vitamin activity comparable to niacin. Furthermore, research and chemistry-based resources use the terms nicotinic acid and nicotinamide; whereas pharmacy resources use niacin and niacinamide. For example, enzyme nomenclature uses the former, whereas facts and comparisons uses the latter. This section uses each term in its best context.
The synthesis and structural identification of niacin actually predated its discovery as a vitamin. Niacin had been prepared from nicotine since at least 1898. Niacin had also been purified and identified in 1913 as a component of yeast and rice polishings by Funk151 in his quest to discover the vitamin later called thiamine. The discovery of niacin as a vitamin was complicated, as discussed under vitamin B2, by the early misconception that animal forms of pellagra and the human form were caused by the same vitamin deficiency. Ultimately, it was the study of human pellagra and black tongue disease in dogs, also caused by niacin deficiency,152 that led to the discovery of niacin as a vitamin. In September of 1937, Elvehjem et al.153 were the first to prove the link between niacin and black tongue disease in dogs. Clinical studies using niacin in human pellagra patients followed, and the first published report of success was in November of 1937.154 Niacin can be synthesized by humans into its active forms, nicotinamide adenine dinucleotide (NAD+) and nicotinamide adenine dinucleotide phosphate (NADP+), from the amino acid, tryptophan. Consequently, and analogous to vitamin D, niacin is not a true vitamin by the classic definition of the term. However, because of inherent inefficiencies in the conversion, the use of dietary tryptophan as a replacement for niacin can result in tryptophan deficiency for protein synthesis. This results in a requirement for niacin in the diet as a vitamin or a diet abnormally high in tryptophan. To compensate for this conversion, the DRIs for niacin are expressed in niacin equivalents (NE) that allows for 60 mg of dietary tryptophan to equal 1-mg niacin. Niacin or niacin derived nu-cleotides can also be synthesized using aspartic acid in plants and microorganisms
Natural sources of niacin include meat (beef, pork, chicken, lamb), fish, and whole grains. As with other B vitamins, fortified ready-to-eat cereals are an important source in developed countries such as the United States.
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